Low nonlinear distortion, high efficiency, and a wider bandwidth are required for a power amplifier.
Today, in general, high efficiency is realized by a Doherty amplifier and distortion is compensated for by a DPD (Digital Pre-Distorter).
For example, a signal with a high PAPR (Peak to Average Power Ratio), such as the WCDMA (Wideband Code Division Multiple Access) method or the OFDM (Orthogonal Frequency Division Multiplex) method, is used for third-generation or later mobile phones, in which case the system bandwidth is several dozen MHz.
A still wider bandwidth signal is used for the fourth generation. However, because a Doherty amplifier requires a back-off according to the PAPR, there is a limit to increasing the efficiency. In addition, load modulation with the use of a quarter-wave line makes it difficult to achieve a wider bandwidth.
To address this problem, study has been conducted for a system that allows the operation to be performed continually in a near-saturation condition by controlling the power-supply voltage of the amplifier according to the amplitude of the signal to be amplified. Such a method achieves high efficiency because it eliminates the need for a back-off in an ideal condition and, at the same time, achieves a wider bandwidth because it does not depend on the frequency of the signal to be amplified. Typical technologies of this method include ET (Envelope Tracking) and EER (Envelope Elimination and Restoration).
FIG. 10 is a diagram showing an example of the configuration of an ET amplifier (ET mode amplifier) in the analog mode.
The ET amplifier in this example includes a power amplifier unit (PA: Power Amplifier) 101, an amplitude detection unit 102, and a PA power supply unit 103.
An example of the operation performed by the ET amplifier in this example is described below.
The input signal is an RF signal generated by superimposing a modulated signal with a high PAPR, such as the CDMA signal or the OFDM signal, on the carrier wave with a radio frequency (RF: Radio Frequency).
The amplitude detection unit 102 receives the input signal and detects its amplitude. The PA power supply unit 103 controls the magnitude of the power supply voltage, supplied to the power amplifier unit 101, according to the magnitude of the detected amplitude. The power amplifier unit 101 receives the input signal, amplifies the power, and outputs the output signal (amplified signal). At this time, a delay circuit (not shown) is inserted as necessary on the receiving side or the power supply side of the power amplifier unit 101 to adjust the time so that the power amplifier unit 101 can supply power corresponding to the amplitude of the input signal.
As an example of the implementation of this amplifier, a diode detector is used for the amplitude detection unit 102, a class-D amplifier is used for the PA power supply unit 103, and a field effect transistor (FET: Field Effect Transistor) or a bipolar transistor is used for the power amplifier unit 101. Other known technologies are used to implement this amplifier.
FIG. 11 is a diagram showing an example of the comparison between the conventional method and the ET method.
In the graph shown in FIG. 11, the horizontal axis indicates the time (Time) and the vertical axis indicates a drain voltage (VDD), applied to the FET, as an example of an amplifier.
In the conventional method (Conventional), the fixed power supply voltage (Fixed VDD) is applied according to the maximum power while, in the ET method, the power supply voltage according to the amplitude of the input signal (Envelop indicated by the dotted line in FIG. 11) is applied to cause the amplifier to operate continually in the saturated condition.
Therefore, as compared with the conventional method, the ET method can reduce the supply voltage corresponding to the part indicated by the shaded part in FIG. 11. That is, the ET method can reduce the power supplied to the power amplifier unit 101, thus increasing efficiency.
The example of the ET method described above is a method for implementing the amplifier for analog signals. Next, the method for implementing the amplifier for digital signals is described.
FIG. 12 is a diagram showing an example of the configuration of an ET amplifier in the digital method.
The ET amplifier in this example includes a D/A (Digital to Analog) converter 111 corresponding to the I component of a complex signal, a D/A converter 112 corresponding to the Q component of a complex signal, an orthogonal modulation unit 113, a power amplifier unit (PA) 114, an amplitude detection unit 115, a D/A converter 116, and a PA power supply unit 117.
In the ET amplifier in the digital method in this example, the amplitude detection unit 115, PA power supply unit 117, and power amplifier unit 114 have the function similar to the processing units corresponding to the ET amplifier in the analog method shown in FIG. 10.
The ET amplifier in the digital method in this example is different from the ET amplifier in the analog method in that the ET amplifier includes three D/A converters 111, 112, and 116 and the orthogonal modulation unit 113.
An example of the operation performed by the ET amplifier in this example is described below.
The input signal is a digital signal composed of the I phase (I component), represented as I(t), and the Q phase (Q component) represented as Q(t). I(t) and Q(t) are each a function of time t.
The amplitude detection unit 115, provided in the digital unit, uses (Expression 1) to calculate the instantaneous amplitude Env(t) based on the input signals I(t) and Q(t). Env(t) is a function of time t.[MATH. 1]Env(t)=√{square root over (I2(t)+Q2(t))}{square root over (I2(t)+Q2(t))}  (Expression 1)
The output signal from the amplitude detection unit 115 (signal corresponding to instantaneous amplitude Env(t)) is converted from the digital signal to the analog signal via the D/A converter 116, and the converted signal is input to the PA power supply unit 117.
The PA power supply unit 117 performs the processing equivalent to that in the analog method shown in FIG. 10.
The input signals I(t) and Q(t) are converted from the digital signal to the analog signal via the two D/A converters 111 and 112. The converted signals are orthogonally modulated by the orthogonal modulation unit 113, and the power of the resulting orthogonally-modulated signal is amplified by the power amplifier unit 114.
Usually, a frequency converter (up converter), not shown, is used depending upon the radio frequency. For example, a frequency converter (up converter) is provided between the orthogonal modulation unit 113 and the power amplifier unit 114.